High stability and low process sensitivity are critical requirements for thick film resistors in microcircuit applications. In particular, it is necessary that resistance (Rav) of a resistor be stable over a wide range of temperature conditions. Thus, the thermal coefficient of resistance (TCR) is a critical variable in any thick film resistor. Because thick film resistor compositions are comprised of a functional (conductive) phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistance and TCR.
Heretofore, thick film resistor compositions have usually had a functional phase consisting of noble metal oxides and polyoxides and occasionally base metal oxides and derivatives thereof. However, these materials have had a number of shortcomings when compounded to produce a high resistance film. For example, the noble metals when formulated to obtain suitably low TCR have very poor power handling characteristics. On the other hand, when they are formulated to give good power handling characteristics, the TCR is too negative. Furthermore, when metal oxides such as RuO.sub.2 and polyoxides such as ruthenium pyrochlore are used as the conductive phase for resistors, they must be air fired. Consequently, they cannot be used with more economical base metal terminations. Still further, when base materials such as metal hexaborides are used, it has not been possible to formulate them to obtain high resistance values (e.g., .gtoreq.30k .OMEGA./.quadrature.) without degrading their power handling ability.
Among the base metal materials which have been investigated for use in resistors are tin oxide (SnO.sub.2) doped with other metal oxides such as As.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Sb.sub.2 O.sub.5 and Bi.sub.2 O.sub.3. These materials are disclosed in U.S. Pat. No. 2,490,825 to Mochell and also by D. B. Binns in Transactions of the British Ceramic Society, January 1974, Vol. 73, pp. 7-17. However, these materials are semiconductors, i.e, they have very highly negative TCR values. In Canadian Pat. No. 1,063,796, R. L. Whalers and K. M. Merz disclose the use of resistors based upon SnO.sub.2 and Ta.sub.2 O.sub.5 which have very highly negative TCR values at high resistances. In addition, these latter materials require processing temperatures of at least 1,000.degree. C.
More recently, in U.S. Pat. No. 4,548,741, Hormadaly disclosed a new class of thick film resistors based upon a conductive phase containing an admixture of finely divided particles of SnO.sub.2 and a pyrochlore corresponding to the formula: EQU Sn.sup.2+.sub.2-x Ta.sub.y.sbsb.3 Nb.sub.y.sbsb.2 Sn.sup.4+.sub.y.sbsb.1 O.sub.7-x-y.sbsb.1.sub./2
wherein PA0 (a) 50-60% wt. of a conductive phase comprising 5-80% wt. of a pyrochlore corresponding to the formula: EQU Sn.sup.2+.sub.2-x Ta.sub.y.sbsb.3 Nb.sub.y.sbsb.2 Sn.sup.4+.sub.y.sbsb.1 O.sub.7-x-y.sbsb.1.sub./2 PA0 wherein PA0 20 to 95% wt. SnO.sub.2, basis pyrochlore and SnO.sub.2 PA0 (b) 50-40% wt. of a borosilicate glass composition which is substantially free of Bi, Cd and Pb comprising by mole %: PA0 (c) 0-10.0% wt. copper oxide adsorbed on the pyrochlore and glass particles, and PA0 (d) 0-10.0% wt. finely divided copper oxide particles admixed with the pyrochlore and glass particles, the total copper oxide in (c) and (d) being at least 0.05% wt. but no more than 10.0% wt. basis total solids.
x=0-0.55 PA1 y.sub.3 =0-2 PA1 y.sub.2 =0-2 PA1 y.sub.1 =0-0.5 and PA1 y.sub.1 +y.sub.2 +y.sub.3 =2, PA1 x=0-0.55 PA1 y.sub.3 =0-2 PA1 y.sub.2 =0-2 PA1 y.sub.1 =0-0.5 and PA1 y.sub.1 +y.sub.2 +y.sub.3 =2, and PA1 (1) 50-85% of a material selected from the group consisting of 25-50% B.sub.2 O.sub.3, 15-40% SiO.sub.2 and 0.1-5% SnO.sub.2 and mixtures thereof and PA1 (2) 50-15% of a material selected from the group consisting of 10-30% BaO, 0-12% MgO, 1-10% NiO and mixtures thereof, further characterized in that PA1 n=number of samples PA1 CV=(.sigma./R).times.100 (%)
the amount of SnO.sub.2 being from 20 to 95% by weight of the admixture. These resistors have been especially successful for applications in resistors of 30k .OMEGA./.quadrature. to 30M .OMEGA./.quadrature.. Nevertheless, despite such advances in the resistor art, there exists an unmet need for economical resistor materials which will give small negative TCR values and preferably even slightly positive TCR values in the range of 5k .OMEGA./.quadrature. to 100k .OMEGA./.quadrature.. Such materials are especially needed for high reliability electronic network applications.